There are many procedures for the production of acrylonitrile. This compound is one of the most important organic chemical intermediates available. It is a major intermediate in the manufacture of a wide range of products, plastics, synthetic rubber, synthetic fibers, soil conditioners and the like. For many uses, acrylonitrile must be of high purity and, for this reason, strict specifications must be met in the commercial manufacture of acrylonitrile. Each of the commercial procedures used for the preparation of acrylonitrile produces its own set of impurities and by-products and each presents its own problems of purification. (See U.S. Pat. No. 3,459,639.)
Different processes for making acrylonitrile result in the formation of different by-product contaminants. Accordingly, different proce dures may be required to remove the contaminants and purify the acrylonitrile. Due to such constraints, any given acrylonitrile purification process is not likely to be universally interchangeable with respect to its usefulness in the purification of all acrylonitrile containing compositions.
As indicated in U.S. Pat. No. 4,404,064, one very good and commercially practiced method of producing olefinically unsaturated nitrites is the catalytic reaction of ammonia and an olefin. For example, acrylonitrile and methacrylonitrile may be produced by the vapor phase catalytic oxidation of propylene and isobutylene respectively, in the presence of ammonia. In these processes, significant amounts of impurities are produced. The production of acrylonitrile from ammonia and propylene results in the formation of significant quantities of acetoni trile., propionitrile, acetone and the like. It is necessary to remove these by-product impurities to produce an unsaturated nitrite suitable for polymerization to other products.
U.S. Pat. No. 3,459,639 to Borrel et al discloses a process for the purification of a complex mixture of acrylonitrile, acetonitrile and other materials formed in the vapor phase conversion of acrolein or propylene to acrylonitrile over a catalyst in the presence of ammonia and oxygen. Separation of acrylonitrile from acetonitrile is accomplished by extractive distillation using deionized water at a pH of at least 5 and preferably 5–7 with the introduction of an alkaline-agent to the distillation mixture.
U.S. Pat. No. 4,377,444 to Wu relates to the recovery and purification of olefinic nitrites and more particularly pertains to an improved process for the recovery and purification of olefinic nitrites, such as methacrylonitrile and acrylonitrile, produced by the ammoxidation of isobutylene and propylene from mixtures of said olefinic nitrites with such materials as acetonitrile, hydrogen cyanide, propionitrile, butyronitrile, methacrolein, acrolein, acetone, acetaldehyde, etc. Wu points out that when an olefin, such as isobutylene or propylene, is allowed to react with ammonia and molecular oxygen in the vapor phase at elevated temperatures and in the presence of an ammoxidation catalyst, the corresponding olefinic nitrites, such as methacrylonitrile and acrylonitrile, are produced along with varying amounts of by-products of the ammoxidation reaction including acetonitrile, hydrogen cyanide, propionitrile, butyronitrile, methacrolein, acrolein, acetone, acetaldehyde, and mixtures of the desired olefinic nitrile, and some of these by-products appear in the ammoxidation reactor effluent. In accordance with the Wu process, the products of the ammoxidation reaction are recovered in a first step by absorption in water during which step some heavy or high-boiling organic compounds are formed through polymerization, condensation, etc., of some of the lighter organic products. Accordingly, the Wu process is an improved method for separating the olefinic nitrites from the by-products formed in the ammoxidation reaction as well as from the heavy organic compounds.
The process disclosed in U.S. Pat. No . 3,051,630 to Hadley et al also relates to the purification of acrylonitrile. However, this process is particularly applicable to the purification of acrylonitrile produced by the catalytic vapor phase reaction of acrolein with ammonia and molecular oxygen. In such reactions, the crude acrylonitrile is usually recovered in the form of a dilute aqueous solution, which also contains varying amounts of acrolein and hydrogen cyanide, by contacting the gaseous reaction product with water, preferably after neutralization of any unreacted ammonia.
The processes mentioned above, mostly deal with the purification of acrylonitrile from the impurities formed during the manufacture of acrylonitrile.
The process disclosed in PCT Patent publication number WO 88/01263 to Hallenburg et al [Lubrizol Corp. USA] relates to a process for recovering and purifying unreacted acrylonitrile from contaminants when excess amounts of acrylonitrile are reacted with another reactant to produce a product such as 2-acrylamido-2-methyl propane sulfonic acid. According to this process, the unreacted acrylonitrile containing contaminants such as the residual acids is first neutralized by adding a base such as NaOH, NH3, Ca(OH)2 or lime (i.e., calcium oxide) and/or mixtures thereof which react with contaminant acids present forming various salts, e.g., calcium salts. Hallenburg points out that lime is the preferred base and is used in the presence of a catalytic amount of water. The neutral salts which are formed as solids precipitate out or can be separated away along with any unreacted lime by conventional means. At this point, the acrylonitrile product still contains contaminants, which would interfere with the use of the acrylonitrile in the synthesis of other materials such as 2-acrylamido-2-methyl propane sulfonic acid. Accordingly, after the removal of the neutral salts, the contaminated acrylonitrile is preheated to about 43° C. to 60° C. in a heat exchanger under vacuum. This preheated material is then transferred to a lower portion of a distillation tower. The tower is comprised of a lower opening, a plurality of distillation trays and an upper opening, with the tower being maintained under vac uum. A majority of the heated material entering the distillation tower falls to the bottom of the tower whereas a small amount of purified acrylonitrile rises to the top of the tower and is evacuated therefrom via a pressure differential. The heated material falling to the bottom of the distillation tower is divided into two portions with the major portion being returned to the heat exchanger and a minor portion is transferred to a thin film evaporator which is maintained under vacuum at a temperature above the melting point and below about 78° C. and preferably in the range of about 49° C. to about 63° C. Within the thin film evaporator, contaminated wastes containing relatively high amounts of residual acids fall to the bottom of the evaporator and a partially purified acrylonitrile product is removed from the top of the evaporator and transferred to the distillation tower. A highly purified acrylonitrile product is removed from the top opening of the distillation tower. Contaminants within the distillation tower continue to fall to the bottom of the tower and are removed. The purified acrylonitrile product removed from the upper portions of the distillation tower contains negligible amounts of contaminants such as residual acids and acrylamides.
Although this process is good for recovering and purifying the excess unreacted acrylontrile from the contaminants generated during manufacture of products such as 2-acrylamido-2-methyl propane sulfonic acid, it has following disadvantages.    1. The unreacted acrylonitrile needs to be first neutralized with bases like lime. This invariably generates slurry of the salts of lime in acrylonitrile. The separation of these salts by conventional means like filtration is difficult and needs a special hardware for closed handling to avoid human exposure to acrylonitrile vapors. If settling and decantation method is used, then it can pose the difficulties in disposal of wet cake loaded with hazardous acrylonitrile.    2. For proper neutralization by lime, water in catalytic proportions is essential. This calls for addition of water to acrylonitrile. Thus the acrylonitrile recovered by this method would contain water in dissolved state as per the solubility characteristics. On the other hand, the moisture content of more than 0.2% by weight in acrylonitrile drastically reduces the yield during the process of manufacture of materials such as 2-acrylamido-2-methyl propane sulfonic acid. Therefore this recovered acrylonitrile has to be further dried by a suitable method like adsorption on molecular sieves etc., before it is used as a reactant in the process. Similarly, the commercial grade acrylonitrile also contains about 0.5% [by wt] of moisture. The moisture content has to be reduced to below 0.2% [by wt] by the methods like adsorption on molecular sieves etc., before this acrylonitrile is used for the manufacture of materials such as 2-acrylamido-2-methyl propane sulfonic acid. Such adsorption processes invariably pose the difficulties in disposal of acrylonitrile vapors during the regeneration cycle.    3. In the process disclosed in the above patent, acrylonitrile is continuously in contact with the contaminants either in the heat exchanger or in the thin film evaporator, due to recirculation and longer residence time distributions. This may induce the polymerization of acrylonitrile and so loss of acrylonitrile. Similarly fouling of heat transfer surfaces and equipment also may not be completely avoided.    4. The process needs a separate hardware for minimizing the moisture content in the unreacted acrylonitrile recovered by above method as well as that in the fresh make-up.    5. The longer residence time in the process imposes restriction on operating temperature for preheating and vaporization of contaminated acrylonitrile within a range of 43° C. to 60° C.
In order to synthesize 2-acrylamido-2-methyl propane sulfonic acid, excess amounts of acrylonitrile are combined with sulfuric acid and isobutene. The resulting reaction product includes 2-acrylamido-2-methyl propane sulfonic acid along with substantial amounts of unreacted acrylonitrile and other by-products. The 2-acrylamido-2-methyl propane sulfonic acid can be separated away leaving the acrylonitrile present along with various residual acids, acrylamides, and other by-product contaminants. The contaminants are present in an amount of about 1–2 percent by weight based on the weight of the composition.
The acid contaminants include sulfuric acid, isobutylene monosulfonic acid, isobutyl ene disulfonic acid and small amounts of 2-acrylamido-2-methyl propane sulfonic acid, t-butyl acrylamide and acrylamide. If the acrylonitrile containing the 1 to 2 percent contaminants such as the residual acids is reused for the production of the 2-acrylamido-2-methyl propane sulfonic acid, the resulting product (i.e., the 2-acrylamido-2-methyl propane sulfonic acid) will have various undesirable characteristics. For example, the resulting product will contain undesirable polymerized material formed by the polymerization of acrylonitrile monomers with contaminant monomers. Accordingly, it is desirable to purify the acrylonitrile before it is reused, and the present invention is directed to such a purification process and the product resulting therefrom.
Accordingly, this purified acrylonitrile itself has the desired average molecular weight and excellent storage stability. Further, the purified acrylonitrile can be utilized in the production of other materials such as being recycled for use in the production of 2-acrylamido-2-methyl propane sulfonic acid, which will also have a high degree of purity. This high degree of purity is particularly important to obtain when eliminating contaminants, which are particularly reactive with the acrylonitrile. Such reactive contaminants react with the acrylonitrile to form water insoluble polymers which cloud up aqueous solutions prepared by including 2-acrylamido2-methyl propane sulfonic acid with water.